Wind energy machines are generally divided into the two categories of cross axis and wind axis devices. A wind axis machine has its blades radiating out from a central axis, like a propeller. This rotational axis must align with the wind for energy production, thus the term wind axis. The cross axis rotor or turbine has its axis of rotation, about which its blades revolve, perpendicular to the wind.
Cross axis rotors can be driven by lift and/or drag forces. Drag is the force on the blade in the general direction of the wind. Drag-driven rotors are relatively inefficient because the drag force aids forward motion only through half a cycle, but impedes on the other half cycle. A wall or shield must be provided to block the incoming wind from slowing the side of the rotor that is advanced toward the wind. Cross axis machines thus lose up to half of their rotor collection area due to shielding requirements, a fact of structural efficiency that has long made wind axis machines, and lift type cross axis machines, favored despite their increased relative cost and complexity.
Thus there presently exists a need for a more efficient cross axis, drag type, wind machine that will enable a more complete use of the wind energy that is normally lost due to shielding.
In the description and claims that follow, the term “vertical” is used to describe a direction that is cross wind, since a vertical axis machine is one common way of configuring such a device. It will be understood to one skilled in the art, however, that with appropriate wind direction alignment, that the device of the invention could be configured with a horizontal, cross wind axis with equivalent function.